1. Field of the Invention
This invention is in the field of methods and apparatus used to cut an exit or side window in a casing, downhole in a well bore, typically for the purpose of drilling a lateral well bore angled away from the main bore.
2. Background Art
Often, it is desired to cut a window in a downhole well casing in an oil or gas well, for the purpose of exiting the casing, to drill a lateral well bore off of the main bore. Typically, multiple mills are used to form such a window having the desired full gage diameter. Often, three separate mills are used, as shown in FIG. 1.
The downhole direction will be shown to the right herein, and the uphole direction to the left, but it should be understood that the borehole could be oriented vertically, horizontally, or anywhere in between. Further, it should be understood that, as used herein, the term “full gage” signifies a window having a diameter which is at least 96% of the full inside diameter of the casing which is being exited. Since the window will have an elliptical shape, the lesser or transverse diameter of such a full gage window will be at least 96% of the full inside diameter of the casing. The greater or longitudinal diameter of the window may be even greater than the inside diameter of the casing. Finally, when the term “separate mills” is used herein, it should be understood that two groups of cutting inserts separated by a substantial axial distance can be considered to constitute two “separate mills”. For example, in some such mills, an axial distance of greater than one inch between groups of inserts could be considered substantial.
Often, the three mills that are typically used to cut such a window are all mounted on a work string WS together, one above the other. The lower or lead mill LM penetrates the casing wall C; then it is followed by the upper mills, commonly referred to as string mills or backup mills B1, B2. In such an apparatus, all three mills LM, B1, B2 typically have the same outer diameter, at least in their original condition. However, by the time the lead mill LM penetrates the casing C, it will usually have a slightly smaller diameter than it originally had, because of damage to the cutting inserts, resulting from severe vibration and impact against the casing C and the whipstock W. The string mills above it are then required to pass through the window in order to open it up to a substantially full gage diameter.
The lead mill LM cuts a relatively small window in the casing C as it is guided therethrough by the guide surface GS of a whipstock W. Rotation of the work string WS then continues as the second mill B1, and eventually the trailing mill B2, pass through the window, as shown in FIG. 2. Each mill slightly enlarges the window as it passes through. Typically, at least two and often three mills must be used, because the cutting inserts on the lead mill LM and even on the second mill B1 often deteriorate rapidly, with the result that the lead mill LM and the second mill B1 lose diameter during the window cutting process. This loss in mill diameter results in the cutting of a window that is slightly smaller than the original diameter of the mills. The mills can not have an original diameter that is larger than the full casing gage, of course. So, the second and third mills B1, B2 must be used to bring the casing window diameter up to full gage.
This typical apparatus having three separate mills on the work string suffers from several disadvantages. First, it is quite costly, because three mills, or at least three spaced apart groups of cutting inserts, are required. Second, the three mills are typically mounted together in a longitudinal arrangement, with the arrays of cutting inserts on the different mills being axially separated by a distance of greater than one inch. This creates a relatively long three mill assembly with a rigid axis. As a result, the apparatus can experience a substantial amount of vibration. Third, because of the long rigid axis of the three mill assembly, it is difficult to turn the mill in a direction away from the whipstock after forming the window and exiting the casing.